It is known that various gut tissues are under endocrine control. The gut hormones that have been most studied are cholecystokinin (CCK), gastrin, and secretin. Studies of the tissue receptors for CCK have been hampered both by the lack of high specific activity biologically active radioiodinated CCK and suitable systems for correlating receptor "binding" with biological responses. The proposed study will use 125I-CCK prepared at high specific activity by conjugation of the hormone with 125I-Bolton-Hunter Reagent, to study CCK receptors in isolated rat pancreatic acini. Preliminary studies with this system have demonstrated the presence of specific high affinity CCK receptors in acini. The characteristics of both the binding and degradation of 125I-CCK to acini will be further studied and correlated with biological activity. Subcellular localization of CCK receptors will be investigated both by subcellular fractionation of tissue after CCK binding and direct binding of CCK to purified organelles. The effect of diet and pathophysiological states including chronic alcohol consumption and experimental diabetes (conditions known to affect exocrine pancreatic secretion) will be studied on both CCK receptor binding and amylase secretion. In addition, CCK receptors will be evaluated in other tissue known to be targets for CCK action (gall bladder, stomach) or which contain large quantities of CCK (brain).